Epitaxial (EPI) quality wafers (hereinafter referred to as “EPI wafers”) are well-known in the art. The term “epitaxial” is defined as the growth of a single-crystal semiconductor film upon a single-crystal substrate. An epitaxial layer has the same crystallographic characteristics as the substrate material. The single-crystalline epitaxial structure comes about when silicon atoms are deposited on a bare silicon wafer in a Chemical Vapor Deposition (“CVD”) reactor. When chemical reactants are controlled and the system parameters are set correctly, the depositing atoms arrive at the wafer surface with sufficient energy to move around on the surface and orient themselves to the crystal arrangement of the wafer atoms.
EPI wafers typically include a heavily doped silicon substrate that provides a very low resistance (typically lower than about 0.001 ohm-sq) and an EPI layer of approximately 1 to 3 microns that is epitaxially grown on the substrate. FIG. 1 illustrates a cross sectional view through an EPI wafer 100 onto which a semiconductor junction diode device can be built. The EPI wafer includes silicon substrate 102 and EPI layer 104. Such devices can be employed to suppress transients of high voltage in a power supply or the like before the transients reach and potentially damage an integrated circuit or similar structure.
One advantage that arises from the use of an EPI wafer to form a semiconductor junction diode device such as depicted in FIG. 1 is that its reverse breakdown voltage, VB, is largely controlled by the dopant concentration of the EPI layer 104, whereas the forward voltage drop of the device is determined by the overall series resistance of the EPI wafer. The overall series resistance of the EPI wafer is in turn largely determined by the resistance of the substrate 102. Since the substrate 102 of an EPI wafer typically has a low resistance, the forward voltage drop of the device can generally be kept desirably low. If on the other hand a non-EPI wafer were employed, the forward voltage would generally be greater since it is difficult to provide non-EPI wafers with a relatively low overall series resistance.
While EPI wafers present the advantage explained above, EPI wafers are very costly. Conventional EPI wafers may run up to approximately $50.00 or more per wafer. Moreover, while several methods have been proposed to obtain EPI wafers at a cheaper cost, the proposed methods are generally limited in some way. For example, less expensive methods can only provide EPI layers that have thicknesses less than 1 micrometer. EPI layers having thicknesses of less than 1 micrometer cause devices (e.g., diodes, transistors) that may be fabricated onto these layers to be limited in performance as these transistors have relatively large junction capacitances. The large junction capacitances primarly arise because since the EPI layer is thin, the junction capacitances extend in the heavily doped substrate.
Accordingly, it would be desirable to provide a relatively inexpensive semiconductor wafer that can be used instead of an EPI wafer to form semiconductor devices with a forward voltage drop and a reverse breakdown voltage that can be tailored within a wide range of values.